1. Field of the Invention
The invention pertains generally to eddy current data obtained from non-destructive examination of a component and more particularly, to a method of combining eddy current data obtained at two different scanning frequencies.
2. Description of the Prior Art
Nondestructive examination of components is carried out in a number of fields and is particularly important in the periodic inspection of steam generator tubing that form part of the primary circuit of a pressurized water reactor nuclear steam supply system. The integrity of the steam generator tubing in the primary circuit of a pressurized water nuclear reactor steam supply system is essential to assure that radioactive coolant from the reactor does not contaminate the secondary side circuit in which it is in heat transfer relationship to create steam to drive a turbine which in turn drives a generator to create electricity. A hot leg of the nuclear reactor primary coolant circuit is connected to one side of a hemispherical plenum on the underside of the steam generator. The hemispherical plenum is divided into two substantially equal parts and bounded on its upper side by a tube sheet. Heat exchanger tubes extend from one side of the hemispherical plenum through the tube sheet into the secondary side in a U-shaped design that terminates through the tube sheet to the other side of the hemispherical plenum. The other side of the hemispherical plenum is connected to a cold leg of the nuclear reactor primary coolant circuit. There are hundreds of tubes within the steam generator communicating between the hot side and the cold side of the plenum. To ensure the integrity of the tubes, periodically, during reactor outages, the plenum is accessed through manways and the tubes inspected. Eddy current probes are inserted into the tubes and the tube position and data read by the eddy current detectors are recorded to identify any flaws that may have developed in the tubes during the preceding service period between inspections. The eddy current data takes the form of signal patterns, which require a great deal of experience to interpret to identify the existence, type and extent of any flaws that may be present in the tubing. If flaws are detected that exceed a given criteria, the corresponding tubing is plugged and thus taken out of service to reduce the likelihood of failure during the forthcoming reactor operating cycle.
Obtaining eddy current data representative of the various kinds of flaws that are likely to be encountered under field conditions, among a background of scattering data and other noise encountered in the field, to train data analysts and test inspection techniques, is extremely difficult and expensive. However, such training is essential to being able to properly interpret eddy current data. Similarly, the testing of inspection techniques is necessary to understand the probability of detecting different types of flaws and the affect the sizing of a flaw has on the various discontinuity responses.
Accordingly, a need exists to acquire eddy current data representative of the detection of a number of different flaws that is suitable for training and qualifying analysts and testing inspection techniques. Desirably, such data should have substantially the same background, scatter and other noise as is encountered in the field.
U.S. Pat. No. 6,823,269 addresses this need by teaching a method for synthesizing eddy current data for this purpose. The steps of the method involve creating a specimen that simulates the component undergoing nondestructive examination with preselected flaws of interest. The specimen is then monitored by an eddy current probe to create a set of eddy current data representative of the flaws detected in the specimen. At least some of the eddy current data collected at a field site is combined with at least some of the eddy current data collected from the specimen to establish a combined data train that reflects the eddy current response to the selected flaws in a background representative of data collected at the field site. Preferably, the eddy current probes used to collect data at the field site and at the specimen are the same type and are operated at the same inspection frequencies and data sampling rates. Furthermore, the patent teaches that it is desirable that the field and specimen data sets are calibrated separately to substantially the same standard so that the signal level and orientation for a given flaw correspond. However, the patent reference recognizes in the real world there will be differences in monitoring conditions between the field data set and the data set obtained from the specimen and states that if there are differences in the inspection conditions, mathematical models can be used to interpolate one or the other of the responses if coil size or inspection frequencies are not identical.
Initially, simple linear combinations of the inspection results from two frequencies were used in an attempt to infer the response at an intermediate frequency. While this produces a result that approximates the desired response it was found that it lacked many of the subtleties present in the original data. It was determined that the shortcoming of the approach was a consequence of the frequency dependent field spread associated with the eddy current coil. This leads to a response being in the lower frequency data at locations where there was none at the higher frequency. A simple combination of the two responses is satisfactory where the two responses overlap but is inadequate where they do not.
Accordingly, a new method is desired that would enable the data union method to be employed with two or more data sets obtained at different frequencies.
Furthermore, such a method is desired that would enable the combination of different data sets obtained at different frequencies without the loss of any of the information in the original data sets.